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300-410 Practice Question: Which THREE symptoms indicate that route…
Which THREE symptoms indicate that route summarization may be causing routing issues in a network? (Choose THREE.)
⚠ Common exam trap
300-410 often tests whether candidates can distinguish summarization symptoms (black holes, suboptimal paths, loops) from unrelated symptoms (SPF CPU spikes, duplicate IPs), so picking CPU or duplicate-IP options reflects a misunderstanding of what summarization actually breaks.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Suboptimal routing paths are observed for certain destinations.
Option A is correct because route summarization advertises a single aggregate prefix that hides the individual component subnets, so a router may forward traffic toward the summary's next hop even when a more optimal path exists for a specific destination, producing suboptimal routing paths. Option B is correct because if a summary route (e.g., 10.0.0.0/16) is advertised but one of its constituent subnets (e.g., 10.0.5.0/24) does not actually exist or is not reachable on the advertising router, packets matching the summary are forwarded and then dropped, creating a black hole. Option C is correct because less-specific summary routes can point traffic back toward a router that only has the summary and not the specific subnet, causing packets to bounce between routers and form a routing loop. Option D is not correct because frequent SPF recalculations and high CPU are typically caused by network instability such as flapping links or adjacencies, not by summarization itself. Option E is not correct because duplicate IP addresses are an addressing/configuration error and are unrelated to route summarization behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Suboptimal routing paths are observed for certain destinations.
Why this is correct
Summarisation advertises a less specific aggregate, so routers may forward traffic toward a summary-advertising neighbour that is not on the optimal path to the actual subnet. The specific longer-match information is hidden, producing suboptimal forwarding for those destinations.
- ✓
Traffic to some subnets is dropped (black hole) even though the summary route exists.
Why this is correct
When a summary is advertised but the advertising router lacks a specific component subnet, traffic matching the aggregate is forwarded to it and then discarded. The summary route exists, yet the destination is unreachable, creating a black hole.
- ✓
Routing loops occur due to less specific summary routes pointing to routers that lack the specific subnet.
Why this is correct
If a router advertises a summary while another router holds a less specific route back toward it, packets can circulate between them. The summary masks the specific subnet, so neither router detects the inconsistency, and forwarding loops persist.
- ✗
High CPU utilization on routers due to frequent SPF calculations.
Why it's wrong here
Frequent SPF recalculations stem from topology churn or flapping links, not summarisation; summarisation actually suppresses such churn by hiding subordinate prefixes. It is tempting because CPU spikes accompany routing instability, but the correct symptom is a black hole or suboptimal path through a summary.
- ✗
Duplicate IP addresses are detected in the network.
Why it's wrong here
Duplicate IP addresses indicate addressing or DHCP misconfiguration, unrelated to summarisation, which aggregates prefixes rather than assigning addresses. It is tempting because both cause reachability failures, but the summarisation symptom is traffic following a less specific route to a null or wrong next hop.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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